פירוש על סוכה 4:10

שאינה מקודשת – if it was not prepared for the Divine service on the Altar. For if he brought it while sanctified, he would invalidate it through leaving it overnight, for all Divine service utensils are sanctified, as it is written (Exodus 30:29): “whatever touches them shall be consecrated,” and all that is holy, the sanctity of the body is invalidated by remaining overnight.

Introduction This mishnah teaches how they would perform the water libation on Shabbat and yet avoid the problem of having to carry the water up from the Shiloah.

היה ממלא מן הכיור – and even though the jug was sanctified as vessels dedicated to the Temple service and which sanctifies its waters, they are not invalidated by remaining overnight, because they would let it down from the evening into a pit, as it states in [Tractate] Yoma (Chapter 3, Mishnah 10): “And he too made a mechanism for the laver so that its water should not be invalidated by being kept overnight.”

As it was performed on weekdays, so was it was performed on Shabbat, save that on the eve of Shabbat he would fill a non-sanctified golden barrel from the Shiloah, and place it in the chamber. Importantly, the mishnah emphasizes that the water libation was performed on Shabbat just as it was performed during the week. I believe that this is frequently one of the ways in which the rabbis emphasized the importance of controversial rituals. It is as if to say that the water libation is so crucial that it is even done on Shabbat. However, they had to make sure that the water was not carried through the public domain on Shabbat. In order to do this they would bring the water up on Friday before Shabbat and leave it in the chamber until the next morning. However, this posed another problem. Water or any other liquid left overnight in a sanctified vessel becomes unfit for use in the Temple. Therefore they would use a golden barrel that had not been consecrated.

שהמים והיין המגולין – that is the reason that if it was revealed, he would fill from the laver and would not offer a libation [from them] for the water [and the wine] that are revealed are invalid for the altar.

If it was poured away or uncovered, he would refill it from the laver, for wine or water which has become uncovered is invalid for the altar. If the water was poured away (perhaps by the Sadducean priest we met yesterday) then it obviously can’t be used. Similarly, if it was left uncovered it also can’t be used. Liquids left uncovered overnight are forbidden for anyone to drink for fear that a snake put venom in them. Since exposed liquids can’t be drunk by people, it is also forbidden to use them on the altar. If the water that they had brought up the day before could not be used, then they would just use water from the laver that was in the Temple.